Prior was the second overall pick in the 2001 draft by the Chicago Cubs, for whom he had some shining moments, but arm injuries curtailed his career. He hasn't pitched in the big leagues since 2006, and has thrown only two innings as high as Triple-A since then.
"He looked good, the velocity's been good," Boston GM Ben Cherington told reporters Wednesday."We're going to leave him down there for a while just to build strength before we send him out to a minor league club. He signed a minor league deal and will keep pitching in extended for a while."
Steven Krasner occasionally covers the Red Sox for ESPNBoston.com.